This invention relates specifically to the formation of ultrapure glass by containerless processing and more specifically to the formation of ultrapure glass rods or glass fibers, including glass containing corrosive substance such as oxides, flourides, or tellurides, by containerless processes.
Also, the elimination of container-induced nucleation would allow deeper undercoolings and much slower critical cooling rates which can extend the range of glass formation to compositions that do not normally form glasses. Consequently, glasses with larger cross-sectional dimensions are possible. Some of these exotic glasses may have unique optical and electroptical properties.
This invention also relates to the production of exotic new glasses from reluctant glass forming compositions containing for example large concentrations of transition earth oxides, alkali earth oxides, or rare earth oxides with unique potential optical properties by containerless processing and more particularly to techniques which minimize heterogeneous surface nucleation thereby allowing slower cooling rates to form glasses than by existing techniques.
The use of optical fibers in information and communication systems is becoming increasingly utilized due to their high quality performance and large capacity transmission properties over long distances. The performance of the optical fibers depends on the purity by which they can be made and is directly proportional to the amount of losses which occur in the transmission of information through the fibers. Low losses and long lengths are very much desired. Silica glass fibers have been made using chemical vapor deposition techniques in a semi-containerless environment.
However, containerless or near containerless processing has the promise of being able to eliminate crucible contaminants from glass melts which would permit the production of ultrapure glasses from other materials. This becomes very important when dealing with materials that are extremely corrosive in the melt or require extremely low impurity fiber optic systems.
For example, halide glass fibers have extremely low theoretical transmission losses, particularly at infrared wavelengths, that are of the order of 10 db/km, being far below that of silica glass fibers that have losses of the order of 2.times.10 db/km. While the silica glass fibers are of sufficiently low loss for a practical optical communication network, there are considerable advantages and cost savings to be obtained with glasses of substantially lower losses, such as the halide based glasses and other exotic glasses such as tellurium based glass fibers.
One prior art example of forming ultrapure glass, including glass formed of corrosive materials, such as halides or tellurides, is disclosed in co-pending U.S. Pat. No. 4,414,164, by the present inventors, and entitled "Containerless High Purity Pulling Process and Apparatus for Glass Fibers," and which is fully incorporated herein by reference.
In this prior art disclosure it has been shown that high-purity fibers can be prepared by levitating a specimen of glass-forming material in a furnace at a temperature sufficiently high to maintain the specimen as a melt and drawing a fiber of the material from the melt. The melt is maintained in a position such that contact with container walls in the furnace is avoided. Levitation can be provided by directing acoustic wave energy so as to produce a stable node in which the melt is supported. Fibers may be drawn from the melt by insertion of a starting strand of wire and then pulling the wire therefrom followed by a glass fiber. Cooling means are employed as necessary to enhance solidification of the fiber and enable effective pulling of the fiber from the melt. This prior art invention provides the primary advantage of minimizing physical contact of the melt with container walls and thus eliminating much of the crucible contaminants which would otherwise produce adverse optical absorption or scattering and limit the propagation of optical signals in the fiber. The prior art also provides the advantage of reducing the surface heterogeneous nucleation rate, minimizing the production of unwanted crystallization. By elimination of heterogeneous nucleation sites, melts can be deeply undercooled, and the range of glass formation can be greatly extended. This suggests the possibility of forming ultrapure glass fibers from a variety of new materials such as halide glasses which cannot be formed into optical fibers by other, more conventional processes. Moreover, very low loss fibers can be produced from more conventional glasses such as those using silica by use of this prior art invention.
However, it has been found that most of the prior art, including the art briefly described above, is limited in the spectrum of glass forming materials with which they can be used and also the variety and quantity of products producible by these known prior art methods.
In summary, most of the pertinent art disclosing methods for forming glasses by semi-containerless processing has been confined to small droplets that are levitated by aerodynamic or acoustic forces and are allowed to either solidify while in levitation, allowed to free fall in a drop tube, or are positioned by small non-contacting forces in an orbiting spacecraft. These techniques generally have not been particularly successful and are inherently limited to fairly small spherical or nearly spherical samples. The size of the glass droplet produced is limited by the nature of the levitation device and by the low conductivity of the melt which limits the cooling rate, and hence the ability to cool the sample sufficiently rapidly to form a glass.
The near spherical shape of containerlessly solidified samples is a result of surface tension forces which control the shape of a free melt. In most cases this is not an optimal shape for usable devices to be made from the sample.
Also, the aerodynamic or acoustic forces used to levitate or position the sample may induce dynamic nucleation and limit the amount of undercooling that can be attained. This ultimately will restrict the limits of glass formation.
A primary object of the present invention is to provide improved methods of forming glass rods and glass fibers using quasi-containerless apparatus.
Another object of the invention is to provide improved methods of forming ultrapure glass rods and glass fibers of materials other than silica such as, for example, halide or telleride based glasses.
A third object of the invention is to provide an improved method of forming extremely low loss glass such as the halide or telleride based glasses as rods for optical components such as laser rods, modulators, Faraday rotators, optical switches, optical filters, optical lenses, fiber optic preforms, and as fibers for use in optical waveguides.
Yet another object is to provide an improved method for forming ultrapure glasses from materials such as halides and tellerides that are easily contaminated because of their corrosiveness in the melted portion of the material which is being formed into a glass.
A fifth object of the invention is to provide a method for making ultrapure glasses in a micro-gravity environment from a wide spectrum of glasses including corrosive glasses such as halides or tellerides.